Cyndi Nyberg
Former EASA Technical Support Specialist
Electric motors often use space heaters to prevent condensation inside the motor when it is not running. In damp areas, they may be necessary to prevent condensation from forming on the windings. They are also important for motors that are on “stand-by” equipment, or motors that are out of service for prolonged periods of time. The function of a space heater is to keep the winding temperature 5o to 10oC above the ambient temperature.
Another type of space heater is the silicone rubber space heater that is applied directly to the winding end turns. One advantage to this type is that they use less power to heat the windings, and they also remain cool to the touch. It is a good idea to insulate between the heater and the winding. It is possible, although rare, for a faulty heater to cause winding failure.
When we repair a motor that has space heaters, or when we want to add them to a motor that does not currently have them, the correct size heater is often unknown. Different manufacturers use different sizes based on the motor type and frame, but there are a couple different equations that can be used to determine the approximate wattage.
Space heaters are rated by watts and voltage (usually 120V or 240V). Space heaters operate at a high surface watt density and therefore at a high temperature. A common practice is to specify a heater at a higher voltage than will actually be supplied; usually half the rated voltage.
This will lower the surface watt density to increase heater life, but the specified watts will need to be higher.
There are two commonly used equations to determine the correct watts for a space heater.
The two equations usually give different results. The two formulas are:
Notice that in the first equation, units are in kilowatts and feet, and in the second, units are given in watts and inches.
The necessary conversions will have to be made.
Depending on the manufacturer, the values you calculate may be very close, or very different.
These are just approximations, and it may be necessary to test the heaters after installation to ensure that the temperature rise is correct.
In general, for small motors, the correct wattage is as high or slightly higher than calculated. For larger motors, you can generally use a wattage in between the two calculated values.
ANSI/EASA AR100
More information on this topic can be found in ANSI/EASA AR100
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